Since you said that the straight stitch looked OK, your machine is probably not far off. I worked in a shop for a while and these were our recommendations when someone was having these sorts of problems:
1 New Needle (make sure that it is in correctly)
2 Re-thread needle and take out bobbin and reload it
3 Remove Needle-plate and vac out fuzz - clean bobbin area as well
4 Use good quality thread (a lot of time I actually use embroidery thread for decorative stitching)
5 Use a light tear away stabilizer
and if all else fails - bring it in,
The needle size will depend on the fabric you are using. You are correct - many machines are finicky and you just have to find what makes them happy. Good Luck